How to count the number of occurrences of an element in a List

I'm pretty sure the static frequency-method in Collections would come in handy here:

int occurrences = Collections.frequency(animals, "bat");

That's how I'd do it anyway. I'm pretty sure this is jdk 1.6 straight up.

This shows, why it is important to "Refer to objects by their interfaces" as described in Effective Java book.

If you code to the implementation and use ArrayList in let's say, 50 places in your code, when you find a good "List" implementation that count the items, you will have to change all those 50 places, and probably you'll have to break your code ( if it is only used by you there is not a big deal, but if it is used by someone else uses, you'll break their code too)

By programming to the interface you can let those 50 places unchanged and replace the implementation from ArrayList to "CountItemsList" (for instance ) or some other class.

Below is a very basic sample on how this could be written. This is only a sample, a production ready List would be much more complicated.

import java.util.*;

public class CountItemsList<E> extends ArrayList<E> { 

    // This is private. It is not visible from outside.
    private Map<E,Integer> count = new HashMap<E,Integer>();

    // There are several entry points to this class
    // this is just to show one of them.
    public boolean add( E element  ) { 
        if( !count.containsKey( element ) ){
            count.put( element, 1 );
        } else { 
            count.put( element, count.get( element ) + 1 );
        }
        return super.add( element );
    }

    // This method belongs to CountItemList interface ( or class ) 
    // to used you have to cast.
    public int getCount( E element ) { 
        if( ! count.containsKey( element ) ) {
            return 0;
        }
        return count.get( element );
    }

    public static void main( String [] args ) { 
        List<String> animals = new CountItemsList<String>();
        animals.add("bat");
        animals.add("owl");
        animals.add("bat");
        animals.add("bat");

        System.out.println( (( CountItemsList<String> )animals).getCount( "bat" ));
    }
}

OO principles applied here: inheritance, polymorphism, abstraction, encapsulation.

Sorry there's no simple method call that can do it. All you'd need to do though is create a map and count frequency with it.

HashMap<String,int> frequencymap = new HashMap<String,int>();
foreach(String a in animals) {
  if(frequencymap.containsKey(a)) {
    frequencymap.put(a, frequencymap.get(a)+1);
  }
  else{ frequencymap.put(a, 1); }
}

There is no native method in Java to do that for you. However, you can use IterableUtils#countMatches() from Apache Commons-Collections to do it for you.

Simple Way to find the occurrence of string value in an array using Java 8 features.

public void checkDuplicateOccurance() {
        List<String> duplicateList = new ArrayList<String>();
        duplicateList.add("Cat");
        duplicateList.add("Dog");
        duplicateList.add("Cat");
        duplicateList.add("cow");
        duplicateList.add("Cow");
        duplicateList.add("Goat");          
        Map<String, Long> couterMap = duplicateList.stream().collect(Collectors.groupingBy(e -> e.toString(),Collectors.counting()));
        System.out.println(couterMap);
    }

Output : {Cat=2, Goat=1, Cow=1, cow=1, Dog=1}

You can notice "Cow" and cow are not considered as same string, in case you required it under same count, use .toLowerCase(). Please find the snippet below for the same.

Map<String, Long> couterMap = duplicateList.stream().collect(Collectors.groupingBy(e -> e.toString().toLowerCase(),Collectors.counting()));

Output : {cat=2, cow=2, goat=1, dog=1}

To achieve that one can do it in several ways, namely:

Methods that return the number of occurrence of a single element:

Collection Frequency

Collections.frequency(animals, "bat");

Java Stream:

Filter

animals.stream().filter("bat"::equals).count();

Just iteration thought the list

public static long manually(Collection<?> c, Object o){
    int count = 0;
    for(Object e : c)
        if(e.equals(o))
            count++;
    return count;
}

Methods that create a map of frequencies:

Collectors.groupingBy

Map<String, Long> counts = 
       animals.stream()
              .collect(Collectors.groupingBy(Function.identity(), Collectors.counting()));

merge

Map<String, Long> map = new HashMap<>();
c.forEach(e -> map.merge(e, 1L, Long::sum));

Manually

Map<String, Integer> mp = new HashMap<>();
        animals.forEach(animal -> mp.compute(animal, (k, v) -> (v == null) ? 1 : v + 1));

A running example with all the methods:

import java.util.*;
import java.util.function.Function;
import java.util.stream.Collectors;

public class Frequency {

    public static int frequency(Collection<?> c, Object o){
        return Collections.frequency(c, o);
    }

    public static long filter(Collection<?> c, Object o){
        return c.stream().filter(o::equals).count();
    }

    public static long manually(Collection<?> c, Object o){
        int count = 0;
        for(Object e : c)
            if(e.equals(o))
                count++;
        return count;
    }

    public static Map<?, Long> mapGroupBy(Collection<?> c){
        return c.stream()
                .collect(Collectors.groupingBy(Function.identity() , Collectors.counting()));
    }

    public static Map<Object, Long> mapMerge(Collection<?> c){
        Map<Object, Long> map = new HashMap<>();
        c.forEach(e -> map.merge(e, 1L, Long::sum));
        return map;
    }

    public static Map<Object, Long> manualMap(Collection<?> c){
        Map<Object, Long> map = new HashMap<>();
        c.forEach(e -> map.compute(e, (k, v) -> (v == null) ? 1 : v + 1));
        return map;
    }


    public static void main(String[] args){
        List<String> animals = new ArrayList<>();
        animals.add("bat");
        animals.add("owl");
        animals.add("bat");
        animals.add("bat");

        System.out.println(frequency(animals, "bat"));
        System.out.println(filter(animals,"bat"));
        System.out.println(manually(animals,"bat"));
        mapGroupBy(animals).forEach((k, v) -> System.out.println(k + " -> "+v));
        mapMerge(animals).forEach((k, v) -> System.out.println(k + " -> "+v));
        manualMap(animals).forEach((k, v) -> System.out.println(k + " -> "+v));
    }
}

The methods name should have reflected what those methods are doing, however, I used the name to reflect the approach being used instead (given that in the current context it is okey).

I wonder, why you can't use that Google's Collection API with JDK 1.6. Does it say so? I think you can, there should not be any compatibility issues, as it is built for a lower version. The case would have been different if that were built for 1.6 and you are running 1.5.

Am I wrong somewhere?

A slightly more efficient approach might be

Map<String, AtomicInteger> instances = new HashMap<String, AtomicInteger>();

void add(String name) {
     AtomicInteger value = instances.get(name);
     if (value == null) 
        instances.put(name, new AtomicInteger(1));
     else
        value.incrementAndGet();
}

What you want is a Bag - which is like a set but also counts the number of occurances. Unfortunately the java Collections framework - great as they are dont have a Bag impl. For that one must use the Apache Common Collection link text

So do it the old fashioned way and roll your own:

Map<String, Integer> instances = new HashMap<String, Integer>();

void add(String name) {
     Integer value = instances.get(name);
     if (value == null) {
        value = new Integer(0);
        instances.put(name, value);
     }
     instances.put(name, value++);
}

Put the elements of the arraylist in the hashMap to count the frequency.

package traversal;

import java.util.ArrayList;
import java.util.List;

public class Occurrance {
    static int count;

    public static void main(String[] args) {
        List<String> ls = new ArrayList<String>();
        ls.add("aa");
        ls.add("aa");
        ls.add("bb");
        ls.add("cc");
        ls.add("dd");
        ls.add("ee");
        ls.add("ee");
        ls.add("aa");
        ls.add("aa");

        for (int i = 0; i < ls.size(); i++) {
            if (ls.get(i) == "aa") {
                count = count + 1;
            }
        }
        System.out.println(count);
    }
}

Output: 4

 Integer[] spam = new Integer[]  {1,2,2,3,4};
 List<Integer>   list=Arrays.asList(spam);

System.out.println(list.stream().collect(Collectors.groupingBy(Function.identity(),Collectors.counting())));
System.out.println(list.stream().collect(Collectors.groupingBy(Function.identity(),HashMap::new,Collectors.counting())));
    

output

{1=1, 2=2, 3=1, 4=1}

If you are a user of my ForEach DSL, it can be done with a Count query.

Count<String> query = Count.from(list);
for (Count<Foo> each: query) each.yield = "bat".equals(each.element);
int number = query.result();

Map<String,Integer> hm = new HashMap<String, Integer>();
for(String i : animals) {
    Integer j = hm.get(i);
    hm.put(i,(j==null ? 1 : j+1));
}
for(Map.Entry<String, Integer> val : hm.entrySet()) {
    System.out.println(val.getKey()+" occurs : "+val.getValue()+" times");
}

You can use groupingBy feature of Java 8 for your use case.

import java.util.ArrayList;
import java.util.List;
import java.util.Map;
import java.util.function.Function;
import java.util.stream.Collectors;

public class Test {
    public static void main(String[] args) {
        List<String> animals = new ArrayList<>();

        animals.add("bat");
        animals.add("owl");
        animals.add("bat");
        animals.add("bat");

        Map<String,Long> occurrenceMap =
                animals.stream().collect(Collectors.groupingBy(Function.identity(),Collectors.counting()));
        System.out.println("occurrenceMap:: " + occurrenceMap);
    }
}

Output

occurrenceMap:: {bat=3, owl=1}